The present invention relates to a radio communication apparatus which allows communication by selectively using one of communication bands respectively used for two systems, for example, the GSM 900 (Global System for Mobile communication at 900 MHz) and the DCS 1800 (Digital Cellular System at 1800 MHz).
As is well known, there are radio communication apparatus which allows communication by selectively using one of communication bands respectively used for two radio communication systems. FIG. 1 shows a part of a basic configuration of a radio communication apparatus of this type, and a conventional communication apparatus will be described below with reference to FIG. 1.
An antenna 1 is an aerial capable of transmitting and receiving communication signals in communication bands A and B (hereinafter simply referred to as "band A" and "band B") used in two radio communication systems.
Signals received by the antenna 1 are separated by an antenna duplexer 2. A reception signal in the band A is input to a low-noise amplifier 31, and a reception signal in the band B is input to a low-noise amplifier 32.
The reception signal in the band A is amplified by the low-noise amplifier 31 and is mixed with a first local signal (local oscillation signal) generated by a first synthesizer 18 at a first downconverter 41 to be subjected to frequency conversion into an intermediate frequency.
The frequency-converted reception signal in the band A is input to a first input terminal of a switch 5 as a first reception IF signal.
The reception signal in the band B is amplified by the low-noise amplifier 32 and is mixed with a second local signal (local oscillation signal) generated by a second synthesizer 19 at a second downconverter 42 to be subjected to frequency conversion into an intermediate frequency. The frequency-converted reception signal in the band B is input to a second input terminal of the switch 5 as a second reception IF signal. The frequencies of the first and second local signals are adjusted such that the frequencies of the first and second reception IF signals become the same intermediate frequency.
The switch 5 selects either of signals input to the first and second input terminals, i.e., either the first or second IF signal in accordance with an instruction from a control portion which is not shown and outputs the signal to a band-pass filter 6.
The band-pass filter 6 places a band restriction on the input reception IF signal to eliminate noise components outside a transmission band.
The reception IF signal which has been band-restricted by the band-pass filter 6 is amplified by an amplifier 7 to a level appropriate for demodulation, demodulated by a demodulator 8 thereafter and input to a subsequent signal processor (not shown) through a signal output terminal 10.
The demodulator 8 demodulates the intermediate frequency signal into a baseband signal using a CW (continuous wave) signal generated by an oscillator 9.
A modulator 12 modulates the CW signal generated by the oscillator 9 using a modulation signal input by the signal processor through a signal input terminal 11 to output a transmission IF signal.
The transmission IF signal is amplified by an amplifier 13 to an appropriate level, thereafter supplied to a low-pass filter to eliminate noise components and harmonic components which are unnecessary for transmission, and input to a switch 15.
The switch 15 is switched under control of a control portion which is not shown depending on the transmission band of the transmission IF signal to output the transmission IF signal to a first upconverter 161 or a second downconverter 162 selectively.
The first upconverter 161 mixes the first local signal generated by the first synthesizer 18 with the reception IF signal to convert it into a high frequency signal in the band A.
The high frequency signal in the band A is subjected to power amplification at a first power amplifier 171 and is then emitted into the air through the antenna duplexer 2 and antenna 1.
The second upconverter 162 mixes the second local signal generated by the second synthesizer 19 with the transmission IF signal to convert it into a high frequency signal in the band B.
The high frequency signal in the band B is subjected to power amplification at a second power amplifier 172 and is then emitted into the air through the antenna duplexer 2 and the antenna 1.
The local signals generated by the first synthesizer 18 and second synthesizer 19 are set at frequencies which are required for frequency conversion on the transmission IF signals to covert them into high frequency signal in the bands A and B, respectively.
The above-described configuration allows the conventional radio communication apparatus to perform communication using a communication signal in the band A or B selectively through control over the switches 5 and 15.
In case that the radio communication apparatus having the above-described configuration is a mobile communication apparatus or the like, the quality of communication over the unselected band may be monitored during communication in preparation for hand-over and, alternatively, the quality of communication over the two bands may be monitored during standby.
It is thus common to switch bands for reception quickly when signals in two bands are to be received. When such high speed switching of reception bands is performed, in order to eliminate the influence of insufficient activation of the synthesizers (synthesizers 18 and 19 in FIG. 1), it is necessary to operate the synthesizer associated with the band to be monitored in addition to the synthesizer for the band in communication prior to monitoring or to cause both of the two synthesizers to operate from the beginning.
However, when the synthesizers 18 and 19 which generate variable high frequency local signals are operated, problem arises in that power consumption is increased. This is a serious problem especially for a mobile communication apparatus whose continuous running time is limited.
This problem occurs not only in the reception system but also in the case wherein two transmission systems are run with synthesizers operated in association therewith.
Further, this problem occurs not only in single superheterodyne type radio communication apparatuses as shown in FIG. 1 but also in a radio communication apparatus which comprises at least two synthesizer for receiving signals in two bands simultaneously, such as double superheterodyne radio communication apparatuses and TDD (Time Division Duplex) type radio communication apparatuses.
In a conventional radio communication apparatus, when two reception systems are operated simultaneously or switched at a high speed to be operated in order to monitor two communication bands, a problem has arisen in that power consumption is increased because two synthesizers must always be operated to variably generate high frequency local signals.